1. Field of the Invention
The present invention relates to noise rejection based upon closed loop servo systems, such as those used to control the output power of ion lasers.
2. Description of Related Art
Many ion lasers employ a cathode that is heated by a source of alternating electrical power, such as commonly available through wall outlets. The fields caused in the cathode heated by this alternating current induce a noise on the output beam which is periodic and substantially repeatable and therefore predictable. The tube also produces other noise, most of which is random and unpredictable.
To reduce both kinds of noise in prior art systems, a closed loop servo has been utilized. However, it is often desirable to further reduce the noise, since the noise can interfere with the laser's intended use. Some uses are especially degraded by the presence of periodic noise.
Historically, companies have employed a light detector to detect the light output of the laser. The light output signal is typically fed into a closed loop servo control system which fluctuates the current through the ion laser tube in order to: (1) reach the desired power output level, and (2) reduce unwanted noise superimposed on the laser beam, especially the line related noise caused by the AC heating of the cathode. Throughout the laser industry, the line noise on the beam has been reduced to specified levels ranging from about 0.1% peak to peak, to about 1% peak to peak. However, many applications require greater reduction of noise on the output beam.
There is much work in the field of adaptive noise cancellation, for example, related to noise generated by automobiles, airplanes, and in factories near heavy machinery, and in the field of computer modems. One prior art system for handling predictable noise in servo systems is known as the least-mean-square algorithm, or LMS algorithm. See, Widrow, et al., ADAPTIVE SIGNAL PROCESSING, Prentice-Hall 1985, Ch. 6, pp. 99-101. For instance, the Widrow text describes one technique for cancelling line noise in electrocardiography (see p. 329, et seq.). However, this approach is based on the use of recursive adaptive filters, which are somewhat processing-intensive. Widrow, et al., Ch. 12, pp. 302-331 (see FIG. 12.1 on p. 304, in particular).
In the ion laser field, the ability of a typical laser servo loop is limited further because of the requirement of retrofitability of the laser head with different power supplies in the field. The head and power supply are usually separable from one another. The slope of the laser beam output (light output per amp of current through the tube) is directly reflected in the servo loop, and directly affects the gain/bandwidth of the servo loop for controlling the noise. This slope varies (most importantly) due to: (1) ordinary variability in manufacturing, (2) aging, (3) the power level used by the customer, and (4) optics. Servo loops for commercial laser systems are typically adjusted to work at maximum current with a laser having maximum slope to insure retrofitability. However, a customer may use the system at low power or may have a tube that has lower than optimal slope. The lower slope in the laser tube causes a lesser noise rejection capability in the servo loop. The gain/bandwidth of the resulting closed loop may be reduced by a factor of 10 or more as compared to what could occur if the servo loop were properly adjusted.
Accordingly, it is important to provide a laser system controller which is capable of reducing line frequency noise beyond current standards, and which optimizes performance of servo loops for a wide variety of commercial laser systems.